The present invention pertains to a type of well drilling bit commonly referred to as a "roller cone" bit. This type of bit includes a bit body having a shank with an uppermost threaded pin whereby the bit can be connected to the lower end of a drill string. Several legs, typically three, extend downwardly from the shank. Each leg terminates in a bearing pin structure, and a respective roller cone-type cutter is rotatably mounted on each of these pin structures. The cone cutters, in turn, have teeth, tungsten carbide buttons, or other cutting structures on their outer surfaces. As the drill bit is rotated, and loaded by allowing a portion of the weight of the drill string to bear downwardly thereon, the cone cutters are caused to roll forcibly along the bottom of the borehole, drilling the formation, typically with a crushing type action.
Drilling fluid or mud is circulated downwardly through the drill string and through the bit to cool the cutters and carry the cuttings back upwardly through the well annulus formed between the borehole walls and the drill string.
As is well known, the cone cutters and bearing pin structures on which they are mounted, are provided with rotary bearings, and these bearings must be lubricated. In most of the roller cone bits currently in use today, a lubricant reservoir is provided in the bit body for each leg. A passageway leads from the reservoir to the respective bearing. A mud pressure port is provided through the bit body to expose the reservoir cavity to the pressure prevailing in the well, and a compensator, such as a diaphragm, is disposed in the reservoir, sealing thereacross, between the mud pressure port and the lubricant passage which leads to the bearing, the lubricant itself being located on the same side of the compensator as the lubricant passage. Thus, the pressure in the well will extend the diaphragm or other compensator so as to control the pressure differential thereacross and properly feed the lubricant into the bearing.
It has been found that pressure tends to build up in the lubricant reservoir and passageway as the bit is used. One explanation of this pressure build up is that, under the high temperatures and pressures which prevail downhole, the lubricant expands, and in addition, may begin to undergo a cracking process similar to that which is performed in petrochemical refinement processes. In any event, it is necessary to vent this excess pressure for various reasons, e.g. to prevent damage to the diaphragm, which could be forced through the mud pressure port, for example. On the other hand, it is not feasible to simply use an open vent, as the lubricant would be lost prematurely.
Prior U.S. Pat. No. 3,476,195 deals with the problem of venting excess lubricant pressures, especially in modern bits utilizing O-ring type bearing seals allowing little or no lubricant leakage. However, the system disclosed merely utilized an ordinary tire type pressure relief valve, and that valve was mounted, and as a practical matter, probably had to be mounted, in a special bore specifically formed for that purpose. Other criticisms of this system were that the valve would open at low pressures, thus allowing premature lubricant loss, e.g. due to surge effects while running the bit into the well.
U.S. Pat. No. 4,019,785 shows a similar elongate relief valve mounted directly in the lubricant feed passage of a bit which has no reservoir and compensator.
U.S. Pat. No. 4,161,223 addresses the problems of the system of U.S. Pat. No. 3,476,195 by providing a scheme in which the entire cap for the lubricant reservoir is itself a pressure relief valve. Nevertheless, there are still problems inherent in this system. In the first place, even though it was attempted to provide a valve which would only open at somewhat higher pressures, the fact that the entire reservoir cap itself served as the valve meant that that valve had relatively large surface areas on which the relevant pressures would act. Accordingly, the pressures at which this valve could, with reasonable design criteria, be made to open were still undesirably low. Furthermore, the fact that the entire reservoir cap had to move to open and close the valve allowed a much greater risk of debris entering the valve area, and thereby interfering with proper resealing or reseating of the valve after venting of the excess lubricant pressure.
U.S. Pat. No. 4,276,946 discloses a compensator diaphragm which attempts to eliminate the need for a relief valve by providing a void space.